For a more comprehensive discussion of the prior art, reference should be made to U.S. patent application Ser. No. 09/304,402 entitled, A Method and System for Scalable Near-End Speech Cancellation for Tip and Ring Tone Detectors which is assigned to the assignee of the present application. The Background of the Invention section of U.S. patent application Ser. No. 09/304,402 is herein incorporated by reference for the purpose of illustrating the state of the prior art and for supplementing the prior art discussion included herein.
The deployment of new CLASS services, such as Calling Identity Delivery on Call Waiting (CIDCW) and Call Waiting Deluxe (CWD), has created renewed interest in telephone near-end speech cancellation methods and systems. Once primarily used to enable bi-directional communications on a single twisted pair, speech cancellation systems are now needed to improve the reliability of inband tone signal detection for vertical telephone services. Vertical services, such as CIDCW and CWD, require special Customer Premises Equipment (CPE) that can reliably detect a unique inband tone signal, known as the CPE Alerting Signal (CAS). The CAS is transmitted by central office switching equipment to initiate transfer of the service-related information. Reliable tone detection is critical for CIDCW and CWD CPE because the CAS is the trigger for the CPE to enter data mode and receive the service information. CIDCW, for example, enables a subscriber who is engaged in a telephone conversation to receive the caller ID information associated with a second call on call waiting. The successful reception and display of the waiting caller's information to the subscriber requires the detection and acknowledgement of the CAS by subscriber equipment. Inband tone signal detection is complicated when resistance to speech simulations and detection of alerting signals masked by the subscriber speech are both desired, as is the case for CIDCW, CWD, and any off hook GR-30-CORE, Issue 2, December 1998, entitled "Voiceband Data Transmission Interface", Issue 1, December 1994, services. Based on the prior art, it is known that the performance and reliability of CAS detection methods can be significantly improved by attenuating or canceling near-end speech signals in the receive channel, wherein the CAS is present, prior to signal discrimination by the CAS detector.
Of particularly high interest are a special class of near-end speech cancellation methods and systems that can transparently be inserted in series with the Tip and Ring telephone interface prior to the communications equipment or a typical communications circuit. These systems are herein referred to as Tip and Ring speech cancellation systems or circuits; these special systems differ in two ways from most traditional two-to-four wire hybrid circuits. First, Tip and Ring speech cancellation systems extract a receive signal channel without terminating the telephone line. One of their key functions is to transparently pass the telephone line supervisory, alerting and communications signals to subsequent or subtending communications equipment or circuitry. Traditional telephone hybrid circuits, on the other hand, were designed for end devices that terminate the telephone line. Second, Tip and Ring speech cancellation systems are designed to highly attenuate near-end speech signals to produce a pure receive channel commonly used to feed tone detection and data demodulation circuits. Traditional hybrid circuits, on the other hand, were designed to support bidirectional communications and engineered to only modestly attenuate near-end speech because a controlled sidetone response was desired.
One popular application for Tip and Ring speech cancellation systems is in telephone adjunct devices, such as Type 2 CIDCW adjuncts. A telephone adjunct connects between the telephone line and the customer's existing equipment, such as a standard telephone set. The value of adjunct devices is that they allow the customer to incrementally add support for enhanced services without necessitating the disposal of their existing equipment. In addition, adjuncts allow CPE manufacturers to introduce new service capabilities without incurring the expense of replacing the functionality already present in the customer's existing equipment. Because adjuncts connect in series with the telephone line, adjunct devices need to faithfully pass telephone supervisory signals (i.e., DC signals), alerting signals (ringing) and communications signals (i.e., AC signals) between the telephone line and the subsequent or subtending communications equipment. Consequently, Tip and Ring speech cancellation systems are well suited for adjuncts that employ near-end speech cancellation to improve tone signal detection.
A second popular application for Tip and Ring speech cancellation systems is in convenient front-end solutions that add enhanced service support to existing communications circuits. Faced with demand for shortened development cycles in a highly competitive environment, CPE manufacturers strongly desire to reuse as much of their existing technology as possible in new products. Because they connect directly to the telephone line and transparently pass telephone supervisory, alerting and communications signals, Tip and Ring speech cancellation systems enable CPE manufacturers to introduce enhanced services circuitry in front of their existing communications circuit designs without adversely affecting their performance or creating need for their modification.
A prior art Tip and Ring speech cancellation system 100 is illustrated in FIG. 1. This arrangement employs the fundamental Wheatstone bridge principle as described in Lim et. al., U.S. Pat. No. 5,796,810, Aug. 18, 1998, entitled "Apparatus for Dialing of Caller ID Block Code and Receiving Call Waiting Caller-ID-Signal" (hereinafter Lim). FIG. 1 shows a basic Wheatstone bridging circuit consisting of elements Ra 112, Rb 113, and R3114. The multiple network option 130 that is shown in FIG. 1 is described in my U.S. patent application Ser. No. 09/304,402, and is discussed below. The objective of the Wheatstone bridging circuitry in FIG. 1 is to match the impedance of R3 to that of Z 115, the unknown impedance of a subscriber loop. In William L. Everitt's "Communications Engineering" (McGraw-Hill 1937) (hereinafter Everitt) a Wheatstone bridge is defined to be a physical circuit comprised of two parallel legs wherein each leg contains the series combination of two impedances. For the purposes of discussion, assume that Ra 112 and Z 115, in FIG. 1, comprise the left leg of the Wheatstone bridge and Rb 113 and R3114 comprise the right leg of the bridge. Specifically, Everitt defines a Wheatstone bridge as: (1) a rhombus-like physical circuit; wherein (2) tapping of two output signals, one from the center of each leg of the bridge, points D and E in FIG. 1, is performed; and (3) circuit balance (i.e., no voltage difference) is achieved when the ratio of the impedances of the left leg of the bridge is equal to the ratio of the impedances of the right leg of the bridge; given that (4) the same potential or input signal is applied across both legs of the bridge. The characteristic balance condition occurs when the voltage drops across Z 115 and R3114 are approximately equal both in magnitude and phase.
In accordance with FIG. 1, if the balance network 114 identically matches the impedance of the loop in both magnitude and angle and fixed resistors Ra 112 and Rb 113 are identical, then any near-end speech signals generated by the subsequent communications equipment or circuit appearing at the center points of the bridge will be identical in both magnitude and phase. Under these conditions, the bridge is said to be balanced. Speech cancellation occurs when the signals tapped from the center points of the bridge are processed by a differential amplifier 116. The differential amplifier 116 subtracts the tapped signals from each other and produces a resultant signal wherein near-end speech is canceled. In practice, resistance Rb 113 is scaled by a factor C greater than resistance Ra 112 to reduce the loading effects of the bridge on the Tip and Ring interface. Likewise, the balance network impedance is scaled by the same factor C to maintain a balanced bridge.
As shown in FIG. 1, in accordance with a classic Wheatstone bridge arrangement, the Lim patent comprises a rhombus-like arrangement of resistors wherein Ra 112 and Rb 113 form the lower half of the bridge and resistor R3114 and Z 115, the loop or line impedance, form the upper half of the bridge. As previously stated, a more convenient construct of Lim's Wheatstone bridge for the purposes of this discussion is to visualize Rb 113 and R3114 as forming the right leg of the bridge and Ra 112 and Z 115 as forming the left leg of the bridge. In accordance with the normal operation of the Wheatstone bridge, the signal source 118, as described by Lim, is connected across the left and right legs of the bridge. In accordance with another characteristic of the Wheatstone bridge, Lim describes the tapping of signals from the center of the bridge, points D and E, and a balance condition when Z/Ra=R3/Rb and the same potential is coupled across the left and right legs of the bridge. In Lim the loop impedance Z 115 is assumed to be 600 ohms, Ra is 10 ohms, R3 is 60,000 (60 k) ohms, and Rb is 1 k ohms. Using these values, the bridge in Lim's patent operates like a classic Wheatstone bridge. In this instance, Z/Ra=R3/Rb=60, the signals used for cancellation are tapped from the center of the bridge, and the same potential or voltage is coupled across the left and right legs of the bridge.
The basic line bridge circuit in FIG. 1 performs rather poorly in practice primarily because loop impedances Z 115 vary over a wide range and deviate from 600.OMEGA. in both magnitude and phase. The near-end speech cancellation performance of the basic line bridge circuit in FIG. 1 rapidly degrades as the impedance of the balance network diverges from the loop impedance. Since loop impedances vary in the telecommunications network, multiple balance networks and a network selection method need to be employed as described in U.S. patent application Ser. No. 09/304,402 to achieve high attenuation of near-end speech energy in the resultant receive signal when interconnecting to the PSTN.
Referring to the multiple network option 130 in FIG. 1, far-end signals or those signals incident upon the bridge from the telephone line are reduced in amplitude by the voltage divider formed by elements Ra 112 and Rb 113 and balance network Bn 120 as described in my U.S. patent application Ser. No. 09/304,402; note that Bn 120 in practice would consist of a plurality of similarly positioned balanced networks, however, for simplicity a plurality is not shown. To maintain transparency of the Tip and Ring interface, element Ra 112 is usually chosen to have a small resistance of 10 ohms. Loop current flow proceeds from the Tip lead through the subsequent communications equipment 118 and returns to the Ring lead through element Ra 112. Since the impedances of elements Ra 112 and Rb 113 are a couple orders of magnitude less than the loop impedance and balance network impedance, far-end signals are significantly attenuated and require amplification by the differential amplifier.
One disadvantage of the prior art line bridge circuit in FIG. 1 even when a perfect balance network is employed is that there is an effective loss in the signal-to-noise ratio for far-end signals. The differential amplifier is typically referenced to a system ground located at about point A since the system diode bridge is usually placed across the secondary Tip and Ring interface leads serving the subsequent communications equipment or circuit. Its input signals are derived from resistors Ra 112 and Rb 113 which are substantially smaller in magnitude than the other elements in the voltage divider circuit. As a result, the far-end signals appearing at the input to the differential amplifier 116 are highly attenuated and brought closer to the system noise floor. The differential amplifier subtracts the attenuated far-end signal appearing at the center point of the balance leg from the attenuated far-end signal appearing at the center point of the primary leg containing the element Ra 112. Because these signals are 180 degrees out of phase, the differential amplifier adds these two highly attenuated signals in the presence of system noise. In this environment, the effective signal-to-noise ratio could be quite low. Increased noise in the resultant signal occurs because both noise and recovered far-end signal are amplified by the differential amplifier. This is undesirable since far-end signals need to be cleanly recovered for input to sensitive tone signal detectors and data demodulators 150.
A second disadvantage of the line bridge circuit in FIG. 1 is that it introduces an imbalance in the telephone line with the insertion of element Ra 112 in one lead of the telephone interface. A fundamental principle of the modern telephone network is the concept of a balanced line to reduce the influence of inductive noise. That is, each lead of the Tip and Ring interface ideally has the same impedance to ground. When noise penetrates the twisted pair in a proper design, its influence on each conductor will be similar. If the pair is balanced, the end terminal equipment, such as a telephone, can extract the metallic communications signal (Tip-to-Ring voltages) and reject the common mode noise signals (Tip-to-ground and Ring-to-ground). However, if the impedance of each lead of the telephone interface is different, longitudinal noise currents will be converted to the metallic noise voltages that are superimposed on the metallic communications signals. By inserting element Ra 112 into one lead of the telephone line interface, the prior art line bridge in FIG. 1 creates a line imbalance and the potential for noise current conversion. Since the conversion of longitudinal noise to a metallic signal depends upon the characteristics of the subsequent communications equipment or circuit, the prior art line bridge is only conditionally transparent. In adjunct applications, control over the characteristics of subsequent equipment is not possible. In such applications, a lack of transparency in the prior art line bridge may cause the subsequent communications equipment to malfunction.
Of utility then would be circuitry which overcomes these limitations in the prior art. Specifically, circuitry that does not degrade the signal-to-noise ratio of the far-end signal and that does not introduce an imbalance in the telephone line would be advantageous.